Low range altimeters (LRAs) are used to determine altitude with respect to a surface, such as the ground or terrain beneath an aircraft. LRAs are typically used to aid guidance during approach and landing, especially during low-visibility conditions. In some instances, LRAs are further used as part of a ground proximity warning system (GPWS).
Instrument landing systems often rely on LRAs to aid guidance during approach and landing under critical weather conditions (e.g. very low visibility due to fog, rain, and/or snow). For example, LRAs may be used to provide flare guidance for flight control systems during a Category III C landing (i.e. approach and landing with no runway visual range limitations). Due to the risks involved, even short periods without information from the LRA are highly undesirable.
However, critical weather conditions can result in very significant amounts of water flowing along the underside of the aircraft fuselage where LRA antennas are typically disposed. The flow of water may be further exacerbated during final approach because the aircraft is typically oriented with its nose up during this stage of flight. Current LRA antenna implementations (e.g. narrow band patch antennas) are susceptible to significant signal degradation from heavy water flow. Accordingly, LRA functionality may be limited under critical weather conditions where signal strength and information accuracy are most needed.